config: move user_name/assistant_name to AppConfig (top level)

These are identity settings, not memory-graph settings. Sat inside the
\`memory\` section only because that's where Config started life. Move
to AppConfig alongside the other top-level stuff.

Readers now pull from \`config::app()\` instead of \`config::get()\`.
subconscious/defs.rs's conversation-building pass still needs Config
for surface_conversation_bytes, so both guards coexist there —
AppConfig's guard is dropped before the per-step await loop so we
don't stall the config-watcher's writer.

show_config picks up the two new fields at the top of its output.
Kent's config already has them hoisted to the top level.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/cli/node.rs

@@ -197,7 +197,7 @@ pub async fn cmd_load_context(stats: bool) -> Result<()> {
         return Ok(());
     }
 
-    println!("=== MEMORY SYSTEM ({}) ===", cfg.assistant_name);
+    println!("=== MEMORY SYSTEM ({}) ===", crate::config::app().assistant_name);
 
     if !personality.is_empty() {
         println!("--- personality_nodes ({}) ---", personality.len());